Gas heated steam generator

ABSTRACT

A steam generator which is heated by hot gas and in which the flow of gas can be controlled so that it is divided between a low pressure reheater and a high pressure reheater for optimum efficiency. The gas enters the steam generator at a gas inlet between the high and low pressure reheaters each of which encircles a central flow tube which at one end has a by-pass valve so that the gas divides into two streams, one flowing toward the by-pass valve and then in the opposite direction through said central flow tube, the other stream flowing in said opposite direction whereby the by-pass valve can be adjusted.

1 GAS HEATED STEAM GENERATOR BACKGROUND OF THE INVENTION Some steam generators are supplied heat by hot gas which is heated outside of the steam generator. They differ from steam generators where the combustion takes place within the generator and the products of combustion are gases which are used as a heating medium. An example is found in nuclear power plants where a gas such as helium is used to cool a reactor and then is itself cooled in a steam generator where it heats water to steam which is used to generate electrical power.

One problem which has been created in such steam generators is the inability to adjust the flow of gases over several reheater sections so that each will receive the proper amount of heat for optimum efficiency.

Still another problem is created by the fact that at reduced power levels of the reactor it is necessary to reduce the gas flow through the reactor by decreasing the gas flow rate through the steam generator.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome drawbacks found in the prior art such as those discussed about. Accordingly. a steam generator with an elongated cylindrical shell has a central flow tube coaxially positioned within the shell adjacent an end of the shell, a high pressure reheater encircling said central flow. tube and a low pressure reheater encircling said flow tube and axially spaced from the high pressure reheater, a gas inlet in the shell and intermediate the ends of the central flow tube, an adjustable by-pass valve at one end of the central flow tube, an evaporator section within the shell axially spaced from the low pressure reheater and high pressure reheater so that when gas flows in through the gas inlet it will divide into two streams, one flowing over one of said low pressure reheater and said high pressure reheater and the other stream flowing over the other of said low pressure reheater and said high pressure reheater to reverse its direction and flow through said by-pass valve and central flow tube, the relative gas flow over said low pressure reheater and said high pressure reheater being controlled by adjusting said adjustable by-pass valve and all of said gas eventually flowing over said evaporator section after leaving said shell.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a view partly in section of a steam generator made in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The drawing shows a steam generator indicated generally as which has an elongated shell 12 having a cylindrical side wall portion 14 an upper end portions 16 and a lower end portion 18. A gas inlet 20 is positioned so as to feed hot gas through the side wall portion 14 at a position closer to the end 16 than the end 18. The gas enters at the gas inlet 20 and flows annularly around a central flow tube 22. The gas inlet 20 is positioned intermediate the ends of the central flow tube 22 so that the upper end of the flow tube 22 is close to the upper end 16 of the steam generator 10. After the gas has flowed in through the gas inlet 20 it will flow annularly around the central flow tube 22 and divide into two streams, one stream flowing upward in the space between the central flow tube 22 and the side wall 14 and the other stream flowing downwardly in the space between the central flow tube 22 and the side wall I4. The stream which flows upwardly will flow in that direction until it impinges against the end 16. Then it must reverse its direction and flow down through the central flow tube 22 to leave that tube at the bottom thereof and merge with the stream of gas moving down outside of the central flow tube 22.

The stream which moves upward to impinge against the end I6 heats a low pressure reheater 24 which comprises a plurality of tubes 26 which receive wet steam through a steam line 28 which feeds an annular header 30. The steam tubes 26 project down from the header 30. The steam tubes 26 feed into an annular header 3| which connects with a steam line 32 through which the reheated steam leaves the present steam generator 10.

The steam which flows downward through the space between the central flow tube 22 and the side wall 14 passes over a high pressure reheater 36 which receives steam through a steam line 38 which connects with a header 40 which feeds steam to a plurality of steam tubes 42 which lead up to a header 44 which is connected to a steam line 46 so that steam coming into the present steam generator 10 will flow upwardly in the tubes 42 to be heated and then leave through the steam line 46.

It has been pointed out how it is a problem to properly divide the steam between the low pressure reheater 24 and the high pressure reheater 36. The present invention solves this problem by means of a by-pass control valve 50 whichcomprises a valve plunger 52 con nected on the end ofa valve stem 54 which is threaded at 56 to a sleeve 58 projecting upward from the center of the end 16. With this arrangement turning the stem 54 will raise or lower the plunger 52 to control the amount of gas flowing down into the central flow tube 22 and therefore control the resistance to flow with respect to the gas stream flowing upward between the central flow tube 22 and the side wall portion 14. Preferably, the plunger52 is tapered at its bottom with the tapered portion extending into the top of the central flow tube 22.

After gas coming down through the central flow tube 22 has merged in space 60 with the gas coming downward in the space between the central flow tube 22 and the side wall 14 the gas flows downward over an evaporator and superheater section 62 which receives feed water through a feed water line 64. The water coming in through the feed water supply line 64 enters a header 66 which is annular in configuration and which feeds a plurality of tubes 68 which extend upward to a header 70 which directs superheated steam to the steam line 72.

The gas flowing over the evaporator-superheater tubes 62 then leaves the present steam generator through the gas outlet 76 which is in the center of the lower end 18 of the present steam generator 10. After passing through the gas outlet 76 the gas enters a gas circulation pump 80 which has an impeller 82 driven through a shaft 84 which is turned by a motor 86. The impeller 82 rotates within a pump chamber 88 which is contained within a casing 90. After leaving the circulation pump 80 the gas flows into a duct 94 where the gas leaves the present steam generator 10 and flows toward the location where it is heated such as a nuclear reactor. The source of heat for the gas is not a part of the present invention and therefore is not shown in the drawing.

It has already been explained how it is important that the flow rate of gas through the steam generator must be controllable to allow for varying loads due to the varying power levels within the reactor. To this end the motor 86 is a variable speed motor which allows for varying the speed of the impeller 82 and thus the amount of helium gas which will pass through the present steam generator 10.

[n the event that the variable speed blower is inoperative or otherwise unavailable, a return duct 100 is provided with a valve 102 to allow recirculating some of the gas coming through the duct 94 to a central recirculation duct 104 which extends within and coaxial with the side wall 14. The recirculation duct 104 is encircled by the evaporator superheater tubes 62. When the valve 102 is opened, gas will flow through the return duct 100 to the recirculation duct 104 to merge with the gas coming down after having passed over the evaporator-superheater 62. This will place some back pressure on the gas coming to the evaporator superheater 62 and thus on the gas coming into the gas inlet 20. The result, of course, is the control of the flow rate of gas through the present steam generator 10.

The foregoing describes but one preferred embodiment of the present invention, other embodiments being possible without exceeding the scope thereof as defined in the following claims.

What is claimed is:

l. A gas heated steam generator comprising:

a generally cylindrical shell closed at both ends;

a first reheater section adjacent one end of said shell;

a second reheater section axially spaced from said first heater section;

a central flow tube coaxial with and within said cylindrical shell and extending through said first reheater sectionand said second reheater section; an evaporator spaced adjacent to the other end of said shell;

21 gas inlet in said shell for allowing gas to flow between said first reheater section and said second reheater section and to divide into two streams one flowing upward over said first reheater section to said one end to reverse its direction and to flow down through said central flow tube, the other stream flowing downward over said second reheater section with said streams merging to flow over said evaporator-superheater section;

a gas outlet in the other end of said shell; and

an adjustable valve adjacent to said one end of said shell for adjusting the rate of flow through said central flow tube to thereby control the portion of the gas coming in through said inlet which flows over said first reheater section.

2. The steam generator defined in claim 1 further comprising a variable speed circulation pump, and a duct for directing gas away from said steam generator with said pump connected between said duct and said gas outlet for controlling the amount of gas which will flow through said steam generator.

3. The steam generator defined in claim 1 further comprising a return duct connected with said duct, a recirculation duct connected with said return duct and extending through said evaporator and superheater section, a valve in said return duct for adjusting the flow through said return duct whereby gas can be di rected through said return duct and said recirculation duct to control the amount of gas which will flow through said steam generator.

4. The steam generator defined in claim 3 wherein said recirculation duct is coaxial with said cylindrical shell and said evaporator superheater extends through the annular space between said recirculation duct and said shell. 

1. A gas heated steam generator comprising: a generally cylindrical shell closed at both ends; a first reheater section adjacent one end of said shell; a second reheater section axially spaced from said first heater section; a central flow tube coaxial with and within said cylindrical shell and extending through said first reheater section and said second reheater section; an evaporator spaced adjacent to the other end of said shell; a gas inlet in said shell for allowing gas to flow between said first reheater section and said second reheater section and to divide into two streams one flowing upward over said first reheater section to said one end to reverse its direction and to flow down through said central flow tube, the other stream flowing downward over said second reheater section with said streams merging to flow over said evaporator-superheater section; a gas outlet in the other end of said shell; and an adjustable valve adjacent to said one end of said shell for adjusting the rate of flow through said central flow tube to thereby control the portion of the gas coming in through said inlet which flows over said first reheater section.
 2. The steam generator defined in claim 1 further comprising a variable speed circulation pump, and a duct for directing gas away from said steam generator with said pump connected between said duct and said gas outlet for controlling the amount of gas which will flow through said steam generator.
 3. The steam generator defined in claim 1 further comprising a return duct connected with said duct, a recirculation duct connected with said return duct and extending through said evaporator and superheater section, a valve in said return duct for adjusting the flow through said return duct whereby gas can be directed through said return duct and said recirculation duct to control the amount of gas which will flow through said steam generator.
 4. The steam generator defined in claim 3 wherein said recirculation duct is coaxial with said cylindrical shell and said evaporator superheater extends through the annular space between said recirculation duct and said shell. 